Oil sands are deposits comprised of bitumen, clay, sand and connate water, and make up a significant portion of North America's naturally-occurring petroleum reserves. To produce a marketable hydrocarbon product from the oil sands, the bitumen must be recovered or extracted from the oil sands matrix. Depending on geographic location, bitumen may be recovered by surface mining or in-situ thermal methods, such as steam assisted gravity drainage (SAGD), cyclic steam stimulation (CSS), vapor extraction process (VAPEX), liquid addition to steam for enhancing recovery (LASER) or derivatives thereof.
Because the bitumen itself is a tar-like, highly viscous material, separating it from the sands poses certain practical difficulties. An example of a common extraction technique is known as a water-based extraction process, where hot water, air, and process aides are added to crushed ore at a basic pH to form a slurry. An oil-rich froth “floats” or rises through the slurry as a hydrocarbon phase which can be skimmed off from the top of a separation vessel. The result is an extract that typically comprises two parts: a hydrocarbon phase known as a bitumen froth stream, made up of bitumen, water and fine solids, and an aqueous phase known as extraction tailings, made up of coarse solids, some fine solids, and water. The bitumen froth typically comprises bitumen (approximately 60% by weight), water (approximately 30% by weight), and solids (approximately 10% by weight), and must undergo a froth treatment process to separate the organic content from the water and solid contaminants. Due to its high viscosity, the first step is typically the introduction of a solvent, usually a hydrocarbon solvent such as naphtha or a paraffinic solvent. This step is known as froth separation, and helps to accelerate the separation of solid particles dispersed within the froth by increasing the density differential between the bitumen, water, and solids as well as reducing the viscosity of bitumen. Separation is carried out by any number of methods, such as centrifugation or gravity separation. Paraffinic froth treatment has several advantages over naphtha-based treatment, as discussed in Canadian Patent Nos. 2,149,737 and 2,217,300. One example of a benefit is the partial rejection of asphaltenes: adding a paraffinic solvent to bitumen froth causes some of the asphaltene component of the bitumen extract to precipitate from the froth and consolidate with the solid components, such as minerals and clays. A further benefit of paraffinic froth treatment is that, as a result of the adsorption of water droplets and clays to the hydrophilic sites of the asphaltene molecules, the final bitumen product contains only a small amount of emulsified droplets and clay particles which can be sources of corrosion and catalyst poisoning. The details of one method of paraffinic froth treatment are set out in Canadian Patent No. 2,587,166 to Sury.
The result of the paraffinic froth treatment process is diluted bitumen and a second tailings stream, known as froth treatment tailings, made up of water, solids, and residual hydrocarbon (solvent, rejected asphaltenes, and un-recovered bitumen) which undergo further treatment to prepare the tailings for safe disposal. Dilution water is added to avoid foaming within the TSRU (described below) and also the blockage of associated tubings and internals The first step in this further treatment is to recover solvent through any number of processes known collectively as tailings solvent recovery. Recovered solvent may then be reused in the froth separation process. Tailings from a tailings solvent recovery unit (TSRU), known as TSRU tailings, are then disposed of Table 1 sets out an example of the composition of TSRU tailings:
TABLE 1TSRU Tailings CompositionComponentWeight PercentMaltenes1Asphaltenes5Solvent0Fines6.5Sands3.3Water84.3TOTAL:100
The specific properties of the tailings will vary depending on the extraction method used, but tailings streams are essentially spent water, asphaltenes, unrecovered hydrocarbon, reagents, and waste ore left over once the usable bitumen has been removed.
While effective, the treatment process requires the use of large quantities of heat, solvent, and water in the form of steam and process water (dilution water), which significantly increases the cost associated with recovery of petroleum from the bitumen-laden oil sands.
One known method of recovering the water is to simply direct the TSRU tailings into reservoirs known as tailings ponds, and allow the solid components to settle and separate from the water over time. Residual heat escapes into the atmosphere, while the tailings water is retained for future use, with some loss due to evaporation. This method is not preferred for at least three reasons. First, a significant amount of time is required for most of the solid materials to precipitate out of the tailings by operation of gravity alone. Secondly, it does not allow for the recovery of any of the large amount of energy contained within the tailings stream in the form of heat. The heat lost is high, as tailings dumped into the ponds are at temperatures between 70° C. and 90° C. Thirdly, tailings ponds do not permit recovery of any of the residual hydrocarbon component within the tailings.
Rather than simply disposing of TSRU tailings, it is desirable to recover a portion of the usable components of the TSRU tailings stream to reduce the overall cost of extracting petroleum resources from oil sands and improve the environmental performance. The energy and water recovered can ideally be reused in further steps of the extraction process or recycled to the TSRU to be used as dilution water. This has the advantage of improving the overall energy efficiency of the extraction process. It is further desirable to minimize the volume of tailings that must be disposed. By removing a certain amount of water from the tailings, the streams can be substantially reduced to minerals and unrecovered hydrocarbon.
Several attempts to recover heat, water, and other reagents from tailings streams are known. Methods are disclosed in U.S. Pat. Nos. 4,343,691, 4,561,965 and 4,240,897, all to Minkkinen. These patents are directed to heat and water vapor recovery using a humidification/dehumidification cycle. U.S. Pat. No. 6,358,403 to Brown et al. describes a vacuum flash process used to recover hydrocarbon solvents from heated tailings streams. There has been, however, a lack of success in effective water and energy recovery.